Interface engineering breaks both stability and activity limits of RuO2 for sustainable water oxidation

Designing catalytic materials with enhanced stability and activity is crucial for sustainable electrochemical energy technologies. RuO2 is the most active material for oxygen evolution reaction (OER) in electrolysers aiming at producing ‘green’ hydrogen, however it encounters critical electrochemical oxidation and dissolution issues during reaction. It remains a grand challenge to achieve stable and active RuO2 electrocatalyst as the current strategies usually enhance one of the two properties at the expense of the other. Here, we report breaking the stability and activity limits of RuO2 in neutral and alkaline environments by constructing a RuO2/CoOx interface. We demonstrate that RuO2 can be greatly stabilized on the CoOx substrate to exceed the Pourbaix stability limit of bulk RuO2. This is realized by the preferential oxidation of CoOx during OER and the electron gain of RuO2 through the interface. Besides, a highly active Ru/Co dual-atom site can be generated around the RuO2/CoOx interface to synergistically adsorb the oxygen intermediates, leading to a favourable reaction path. The as-designed RuO2/CoOx catalyst provides an avenue to achieve stable and active materials for sustainable electrochemical energy technologies.

13) Neutral conditions were achieved by the use of PBS buffer, how the presence of phosphate species at the surface of the catalyst (in particular on RuO2 fig 4f) reported through in situ surface enhanced IR spectra interfere with the catalysis process?

More general comments
14) The description of all the materials does not seem very clear and hence rather incomplete. In particular, with the data provided, it seems that the modelling of the RuO2/CoOx hybrid is not in agreement with the experiments. On fig. S5, we can see a classical TEM image of Ru/CoO and EDS elemental mapping showing that Ru is dispersed throughout all the surface of the rod and so I cannot relate to the reported description of the system as islands of RuO2 on a CoOx surface. In particular the HAADF-STEM (and not HADDF-STEM) presented in fig. S14a shows not a significant contrast that should arise from the difference in atomic number between Ru and Co. It is true they are both converted in oxides but still. So I think a HAADF-STEM images of the Ru/CoOx precursor is needed because I have the feeling that all the surface is covered by Ru particles and so the effect of the interface/triple point on the catalytic activity should be limited (cf paragraph "Origin of the enhanced OER activity"). The analysis of the fig 5a is deceiving in my point of view because the figures 2c and 2d are much in favor of a continuous Ru shell at the surface of the CoO rod. Could the authors provide more details on the actual description of their samples and hybrids, with more experimental evidences, since this a new material described? 15) Considering the fact that interfacing of RuO2 with CoOx leads to Ru atoms with a higher electron density (cf fig 1d and 2e), this seems to be good for improving the stability but how is it better for improving the oxidation of water of hydroxide ions? I might miss something but Co charge does not change and the mechanism is said to occur at the interface (Ru/Co dualatom site) between the two materials. 16) Is there any strains or defects induced by the transformation of Ru nanoparticles into RuO2 nanoparticles at the interface between Ru and CoOx because we only see one detail of this interface (same fig S8, S9 and S12)?
17) RuO2 is mainly a crucial catalyst for OER in acidic conditions since very efficient PGM-free catalysts are already available for alkaline OER. I regret that there is no mention of what happens in acidic condition (and the Pourbaix Diagrams in Fig. 1a and 1b are a bit deceiving in this sense because one goes to pH = 0 and the second only to 7: the calculations are not valid when pH<7?) Is this strategy of not applicable at low pH? The system seems anyway to be a bit complex to be included in a real electrolyzing cell and dissolution of the CoOx in low pH for instance may degrade the membrane.
18) Page 7, line 167-169, one can read "This finding is further supported by the in situ IR spectroscopy characterization (Fig. 4e and Supplementary Table 6), which shows a more pronounced HOO* band of RuO2/CoOx in comparison with that of RuO2 (Fig. 4f). These results suggest that the RuO2/CoOx exhibits a different OER mechanism compared with the pristine RuO2." By checking in references 33, 38 and 39 (in the SI), it seems there is no value in the literature corresponding for this vibration in the 1150-1180 cm -1 range which is indicated in the supporting table 6 and highlighted in the figures 4e and 4f. So the assignment of the OOH* band seems doubtful. First the fact that the OOH* intermediate is adsorbed on RuO2/CoOx and not on Pt or Pd should provide a different wavelength number (it seems it is close from 1212 cm -1 in reference 34 in the SI) so why the 1158 and 1177 cm -1 are assigned to this specific vibration because I see no such values in the reference papers cited?
Isotopic measurements with H2 18 O could have been fruitful to gain evidence for this attribution. Does the phrase "the band is more pronounced" suggest that there is a quantitative difference (more or less adsorbates) and so a difference in the OER mechanism ?
control. It would be good if the authors explain if the improvement gained on those local dual sites is enough to surpass the lower activity on the pristine ones.

Response:
Thanks for your valuable comment. The relative low activity of CoOx compared with those of RuO2/CoOx and RuO2 excludes pristine Co site as the most active OER site. We performed additional experiments to verify Ru/Co dual-atom sites as the most active OER sites. First, we deposited RuO2 nanoparticles on carbon black with the same particle size (~2 nm) as that on RuO2/CoOx ( Fig. R1a and R1b). The higher current density of RuO2/CoOx compared with that of RuO2 with the same RuO2mass ( Fig. R1c) indicates the crucial role of the interface in enhancing OER performance of RuO2/CoOx. Second, we prepared RuO2/CoOx with varied RuO2 particle size or loading masses (Fig.   R2a). Assuming either surface Ru site or interfacial Ru-Co dual-atom site as the most active sites, the corresponding site number was calculated and correlated with the measured OER current density at 1.50 VRHE ( Fig. R2b and R2c). An adequate linear relationship was observed between the number of interfacial Ru-Co dual-atom sites and the OER current density (Fig. R2c). These collective results reveals that the interfacial Ru/Co dual-atoms are the most active OER sites on RuO2/CoOx.
We would like to say that although the number of interfacial atoms is relatively small compared with that of surface Ru and Co sites on pristine RuO2 and CoOx, respectively, the OER mechanism on these interfacial sites is different from that on pristine RuO2 and CoOx. This is the origin of the significantly enhanced performance of RuO2/CoOx.

Response:
Sorry for misleading as we didn't give out the definition of the formation energy in the previous version. The "formation energy" was calculated by the following equation:

Response:
We would like to thank the Reviewer for his/her valuable comments and positive recommendation.

General Comments:
However, I think the total novelty of this work is not enough, as a similar approach has been published by Yu et al. (ACS Sustainable Chem. Eng. 2020, 8, 17520−17526)  the authors simply ascribed the improved activity of CoOx−RuO2 (0.26 V overpotential @ 10 mA cm -2 in 1.0 M KOH) to sufficient active sites and binder free of CoOx−RuO2. In our work, we tackle the electrochemical corrosion of RuO2 under OER conditions and achieve record high OER activities under both neutral and alkaline conditions. Moreover, the physical origin of enhanced stability and activity of RuO2 is clearly shown in our work. We note that although their material system is similar with ours, the amorphous structure of CoOx limits the strong atomic and electronic interaction between CoOx substrate and RuO2, which is however the novelty of our work.
Perhaps in the original submission we failed to underline the significance and novelty of this work.
Therefore, we would like to highlight them below as compared to the key points of the works of Yu et al. and other related works: 1. This work represents the first attempt to exceed the Pourbaix stability limit of bulk RuO2 under OER conditions. Although some works have reported enhancing the stability of Ru-based catalysts, there still lacks a general strategy that can fundamentally suppress the electrochemical corrosion of RuO2 under OER conditions. We propose to stabilize RuO2 via constructing stable RuO2/CoOx interface. Specifically, the preferential oxidation of CoOx during OER process and the electron gain of RuO2 through the RuO2/CoOx interface remarkably reduce the driving force of RuO2 dissolution in the hybrid, which makes the stability of ~2 nm RuO2 nonoclusters in the hybrid far beyond the bulk Pourbaix limits of RuO2 (Fig. 1a, b). As a result, RuO2/CoOx hybrid exhibits an outstanding stability for more than 200 h, withstanding anodic potential as high as 2 VRHE with no Ru dissolution being detected, as verified by in situ X-ray photoelectron spectroscopy and inductively coupled plasma mass spectrometry.

Original comment 2-1:
Page 4 "Note that the change in the Co charge at the interface relative to the value in the bulk CoOOH is negligible (Fig. 1f)." Why? The difference of Co bader charge between bulk and interface can be negligible?

Response:
Sorry for unclear expression. We meant that the change of Co charge at the interface relative to the value in CoOOH is ~ 0 (ΔQCo= QCo interface -QCo CoOOH ). Accordingly, to avoid misunderstanding, the related description has been revised in lines 11-12, page 4 in the revised manuscript.
"Note that the Co charge at the interface is almost identical to that in the bulk CoOOH (Fig. 1f)." Original comment 2-2:

Response:
Thanks for your valuable comment. Our EELS measurement ( Fig. 2e-f) shows that the enrichment of Ru charge at the interface will affect the distribution of Ru charge in the bulk and on the surface of RuO2 through continuous Ru-O bonds. The smaller the RuO2 particle size is, the greater the interface effect will be. In our case, the size of RuO2 nanoparticles deposited on CoOx is only 2 nm, which falls within the range of strong interfacial interaction (Chem. Rev. 2018, 118, 4981). This result is supported by the calculated Pourbaix diagram of RuO2/CoOx ( Fig. 1b) that in the OER potential range, the entire RuO2 particle (including the surface) is stabilized on CoOx.
Regarding the active sites, we carefully performed a series of control experiments to verify that the interfacial Ru/Co dual-atom sites are the most active sites for OER. Please see details in Response to your comment 2-5. It is interesting to find that although the number of interfacial atoms is relatively small compared with that of surface Ru and Co sites on pristine RuO2 and CoOx, respectively, the OER mechanism on these interfacial sites is different from that on pristine RuO2 and CoOx. This is the origin of the significantly enhanced performance of RuO2/CoOx.
In response, we have added related discussion in lines 20-21, page 5 in the revised manuscript.
"We note that the enrichment of Ru charge at the interface will affect the distribution of Ru charge in the bulk and on the surface through continuous Ru-O bonds."

Original comment 2-3:
Authors roughly conclude that the EELS analysis supports the calculated charge changes, especially O ions from CoOx to RuO2 via the interface, please clearly describe.

Response:
According to the suggestion of the Reviewer, the related description has been revised in lines 9-21, page 5 in the revised manuscript.
"As illustrated in Fig. 2e

Response:
Thanks for your comment. We meant the different OER mechanism is the different RDS of OER.
where * is the active site. Original comment 2-5: Since the Ru-Co dual-atoms sites are served as co-works active sites in simulations analysis, the evidence of the corresponding experiments discovery cannot be missed.

Response:
Thanks for your kind suggestion. We have performed additional experiments to verify Ru/Co dual-atom sites as the most active OER sites. First, we deposited RuO2 nanoparticles on carbon black with the same particle size (~2 nm) as that on RuO2/CoOx ( Fig. R3a and R3b). The much higher current density of RuO2/CoOx compared with that of RuO2 with the same RuO2-mass ( Fig. R3c) indicates the crucial role of the interface in enhancing OER performance. Moreover, we prepared RuO2/CoOx with varied RuO2 particle size or loading masses (Fig. R4a). Assuming either surface Ru site or interfacial Ru-Co dual-atom site as the most active sites, the corresponding site number was calculated and correlated with the OER current density at 1.50 VRHE ( Fig. R4b and R4c). An adequate linear relationship was observed between the number of interfacial Ru-Co dual-atom site and the OER current density (Fig. R4c). These collective results reveals that the interfacial Ru/Co dual-atoms are the most active OER sites.   Fig. 36)

. An adequate linear relationship was observed between
NRu/Co dual-atom sites and the OER current density (Supplementary Fig. 36c). Therefore, these collective results reveals that the interfacial Ru/Co dual-atoms are the most active OER sites." Original comment 2-6: Page 8 "Relative shift of *OOH bands is observed in the in situ IR spectra of the RuO2/CoOx compared with those of RuO2 (Fig. 4e, f)," Please mark the indicated bands in the corresponding figures.

Response:
Thanks for your kind suggestion. We have marked the indicated bands in the revised Fig. 4e, f. Figure R5. In situ surface-enhanced IR spectra of RuO2/CoOx and RuO2 at different potentials.

Original comment 2-7:
"…, verifying the adsorption configuration of *OO-H…O, which facilitates the stabilization of *OOH at the Ru/Co dual-atom site around the interface (inset of Fig. 5c and Supplementary Fig. 27)." As mentioned above, how to experimentally prove these configurations on Ru-Co dual-sites?

Response:
Thanks for your comment. According to previous work (J. Phys. Chem. A 2018, 122, 4481), in IR spectra, when intermolecular hydrogen bond is formed and stretches the target bond in the probe molecule, the vibrational frequency of the probe molecule will shift toward the low wavenumber direction. Relative*OOH band shift is observed in the in situ IR spectra of the RuO2/CoOx compared with those of RuO2 (Fig. 4e, f), verifying the adsorption configuration of *OO-H…O.
In response, we have added related discussion in lines 15-20, page 8 in the revised manuscript.
"According to previous work 35 , when the intermolecular hydrogen bond stretches the bond in the probe molecule, it will lead to a shift of the stretching vibrational frequency of the probe groups toward the low wavenumber direction in IR spectra. Relative shift of *OOH bands is observed in the in situ IR spectra of the RuO2/CoOx compared with those of RuO2 (Fig. 4e, f), verifying the adsorption configuration of *OO-H…O, which facilitates the stabilization of *OOH at the Ru/Co dual-atom site around the interface (inset of Fig. 5c and Supplementary Fig. 32

Response:
Thanks for your kind suggestion. The activity of CoOx is much lower than that of RuO2/CoOx (Fig. 4a). We compared the performance of RuO2 and RuO2/CoOx with the same RuO2 particle size and RuO2-mass. Please see details in Response to comment 2-5. These collective results confirm the Ru/Co dual-atom site as the most active site for OER. We would like to say that although the number of interfacial atoms is limited compared with that of surface Ru and Co, the OER mechanism on these interfacial sites is different from that on pristine RuO2 and CoOx. This is the origin of the significantly enhanced performance of RuO2/CoOx.

Original comment 2-9:
The information about Ru-O-Co bonds should be additionally characterized and provided.

Response:
Thanks for your kind suggestion. Accordingly, we have characterized the Ru-O-Co bonds by Fourier transform extended X-ray absorption fine structure (FT-EXAFS). As shown in Fig. R6a, a new peak assigned to Ru-O-Co bonds (with shorter Ru-O distance than that in RuO2) appears in the FT-EXAFS spectrum of RuO2/CoOx. In response, we have added Fig. R6 as Supplementary Fig. 10 in the revised supporting information and added the above related discussion in the caption of

Response:
We would like to thank the Reviewer for his/her valuable comments to help us to improve the quality of this manuscript.

Original comment 3-1:
Intrinsic activity of the materials (normalized to the BET/electrochemically active surface area) rather than specific activity should be compared to prove that the electronic effects of the CoOx support rather than enhanced dispersion of the active species (compared to bulk RuO2) are responsible for the activity improvement (RuO2/CoOx vs. CoOx).

Response:
Thanks for your kind suggestion. Accordingly, we have tested the electrochemically active surface areas (ECSAs) of RuO2/CoOx, RuO2 and CoOx (Fig. R7a-d), and normalized their OER current density to the corresponding ECSA to obtain the specific current density (JECSA). As shown in Fig.   R7e, JECSA of RuO2/CoOx is higher than those of RuO2 and CoOx, demonstrating that the electronic effect between RuO2 and CoOx is indeed the reason for the enhanced activity of RuO2/CoOx. 16 Figure R7. In response, we have added Fig. R7 as Supplementary Fig. 27 in the revised supporting information.

Original comment 3-2:
Turnover frequency (P. 7) cannot be calculated based on Ru content only as there are no strong evidences that Ru is the only reaction site.

Response:
Thanks for your valuable comment. We agree with the Reviewer that the TOF should be calculated based on the number of the "real" reaction sites. In our case, the real active sites for RuO2/CoOx are the interfacial Ru/Co dual-atom sites. To verify this point, we have performed additional experiments.
First, we deposited RuO2 nanoparticles on carbon black with the same particle size (~2 nm) as that on RuO2/CoOx ( Fig. R8a and R8b). The higher current density of RuO2/CoOx compared with that of RuO2 with the same RuO2-mass (Fig. R8c) indicates the crucial role of the interface in enhancing OER performance of RuO2/CoOx. Moreover, we prepared RuO2/CoOx with varied RuO2 particle size or loading masses (Fig. R9a). Assuming either surface Ru site or interfacial Ru-Co dual-atom site as the most active sites, the corresponding site number was calculated and correlated with the OER current density at 1.50 VRHE (Fig. R9b and R9c). An adequate linear relationship was observed between the number of interfacial Ru-Co dual-atom sites and the OER current density (Fig. R9c).
These collective results reveals that the interfacial Ru/Co dual-atoms are the most active OER sites.
In the original submission, we calculated the TOF by normalizing the production rate of O2 to the total Ru atoms on RuO2/CoOx. It is an underestimation, however it is generally used in literature (Nat.   in the revised reference list.

Original comment 3-4:
I recommend to refrain from using phrases like "innovatively constructed", "dream RuO2", "unprecedented high stability" to highlight the properties of the material.

Response:
Thanks for your kind suggestion. The related descriptions mentioned by the Reviewer have been revised in the revised manuscript.

Original comment 3-5:
Fig. 1b: Ru species are missing from the low potential/pH region; Fig. 3f: what are the potentials applied?

Response:
Thanks for your kind noticing. "RuO2" has been added in the low potential/pH region in the revised Fig. 1b, and the applied potentials were indicated in the caption of Fig. 3f.

Response:
We would like to thank the Reviewer for his/her valuable comments to help us to improve the quality of this manuscript. We have added additional experimental evidences and descriptions of our samples in the revised manuscript and supporting information. We believe that our strategy proposed in this work is applicable to RuO2 in acidic solution and other highly active materials with unsatisfactory stability if proper substrate is selected. Moreover, we confirm that the structure of our RuO2/CoOx is consistent with the computational model, and the conclusion drawn in this work is trustful. Please see details in our responses to your comments.

Original comment 4-1:
The Moreover, we have carefully checked all the descriptions of "CoOx" and "CoO" in the revised manuscript and supporting information.

Original comment 4-2:
How the value of the concentration for the Pourbaix diagram (10 -6 M) was chosen and why is it similar for the calculation of Co-based, Ru-based and Co/Ru-based materials?

Response:
Thanks for your comment. 10 -6 M is a typical concentration for Pourbaix diagram calculation (Nat. Energy 2017, 2, 17070; Phys. Rev. B 2012, 85, 235438), which represents a considerable dissolution. Therefore, it was chosen for calculation of the three kinds of materials in this work.

Response:
Thanks for your comment. Accordingly

Response:
Thanks for your kind notice. We have revised the related typo mistake in the revised supporting information.

Original comment 4-5:
The authors mentioned two types of substrates:

Original comment 4-6:
We can see some darker circular stains on the CoOx nanorods (fig 2b, fig. S14a), from where does it come? Response:

22
Thanks for your comment. The darker circular stains mentioned by the Reviewer are nanopores (NPs) generated by the release of lattice strain between the template (ZnO) and product (CoO) during the cation exchange process. It is a common structural characteristic in materials prepared by cation exchange method (Chem. Soc. Rev. 2013, 42, 89). Fig. R11 is a TEM image of the nanorod, and the nanopores are indicated. Figure R11. TEM image of as-exchanged CoO nanorod.
In response, we have indicated the nanopores in Supplementary Fig. 17b (original Supplementary Fig. 14a) and added related discussion and the above reference in the caption of Supplementary Fig. 17.

Response:
Thanks for your comment. In previous XRD test, CoO nanorods were stripped from FTO substrate and loaded on CFP for characterization (the signal of the FTO substrate is too strong). According to the comment 4-5 raised by the Reviewer, we realized that it may be a little misleading. Therefore, we have re-tested XRD spectra of CoO and Ru/CoO without any substrate (Fig. R12).
Accordingly, we have replaced the XRD spectra in Supplementary Fig. 3 with the newly tested ones.

Fig S7 shows the electrochemical oxidation of Ru/CoOx between 0.8 V and 1.5 V vs RHE while it is
said to be between 1.0 and 1.5 V in the experimental part? Which one is the good value? It is the same range for RuO2 only? It seems that in one case, the potential is set to a value where the catalytic regime occurs while in the other case no (cf fig. 4a), does it have an impact on the catalysis after wards?

Response:
It is a careless typing, which causes misleading. In fact, the potential range (0.8~1.5 VRHE) is applied to ensure complete transformation of Ru to RuO2 and a narrower potential has no significant impact on the catalytic activity (Fig. R13). Thank you again for finding and pointing our mistake. Accordingly, we have revised the related description in the experimental part.

Original comment 4-9:
It is possible to compare two systems which do not have the same morphology (RuO2 cf. fig 21 and RuO2/ CoOx, cf fig 2) but the authors should give some impacts of the morphology on the electrochemical activity and compare data for a same given mass or given surface area since the RuO2 reference is not a commercial sample.

Response:
Thanks for your valuable comment. We completely agree with the Reviewer that the morphology has an important impact on electrochemical activity of catalysts. Therefore, the size and distribution of RuO2 deposited on carbon black was controlled as identical to those on CoOx (Fig. R14).
Consequently, the current activity difference between RuO2/CoOx and RuO2 can explain the role of RuO2/CoOx interface. Moreover, according to the suggestion of the Reviewer, we have compared the OER performance of RuO2/CoOx and RuO2 with identical RuO2-mass loading. As shown in Fig. R15c, RuO2/CoOx exhibits a much higher current density than RuO2 with the same RuO2-mass loading.
In response, we have added Figs. R14 and R15 in Supplementary Figs. 17, 19, 25, and 34 in the revised supporting information.    In response, we have revised Supplementary Tables 3, 4, and the related labels and descriptions in Supplementary Figs. 24, 25 and 26 in the revised supporting information.

Original comment 4-11:
On fig 3a, it seems that the peak for the C1s is shifted to lower energy while this of Ru 3d (3d5/2) does not move. My question is: how were those spectra calibrated in energy since usually the C1s peak, related to adventitious carbon most of the time, serves as the reference?

Response:
It is a visual illusion by 3D image display in original Fig. 3a. As shown in Fig. R16, C1s peaks do not shift. Accordingly, we have replaced Fig. 3a with Fig. R16 in the revised manuscript. Figure R16. In situ Ru 3d XPS spectra recorded at applied potential during 1.00-2.00 VRHE.

Original comment 4-12:
No deconvolution of the XPS spectra are presented for the Ru 3+ and Ru 4+ ratio determination and even for all the element while a lot of literature exist on that subject. This is a missing aspect. A very good reference for Ru is 10.1002/sia.5852.

Response:
Thanks for your kind suggestion. We have done deconvolution of the Ru XPS spectra (Fig. R17).
In response, we have added Fig. R17 as Supplementary Fig. 14 in the revised supporting information.
Moreover, the reference the Reviewer mentioned has been cited in the caption of Supplementary   Fig. 14. 28 Figure R17. In situ Ru 3d XPS spectra recorded at applied potential during 1.00-2.00 VRHE.
Original comment 4-13: cm -1 . We note that the location of these peaks is different from that of *OOH detected (Fig. 4e, f)

Response:
Thanks for your comment. The dispersion of Ru element throughout the surface mentioned by the Reviewer results from the limited spatial resolution of EDS at low magnification (with large electron beam size). Moreover, we agree with the Reviewer that the contrast of RuO2 and CoOx is indeed not significant in original Supplementary Fig. 14a. This is due to the relatively low resolution of HAADF-STEM image taken during EDS mapping. Fig. R18 shows the same RuO2/CoOx imaged in the TEM mode when searching the sample (not the same area as that in original Supplementary Fig. 14a). As seen, RuO2 "islands" are very clear on CoOx. Figure R18. TEM image of the RuO2/CoOx. 30 We note that the distribution of nanoparticles on the nanorod is three-dimensional, however the TEM/HADDF-STEM image is a two-dimensional projection. The "islands" or "continuous shell" like morphology of nanoparticles depends on the zone axis of CoO when the TEM image is taken. To explain this phenomenon, we construct a structure of Ru/CoO ( Fig. R19a and b are the same model, just viewed from different directions). If the nanorod is situated in the direction shown in Fig. 19a, the "islands" can be clearly seen (This is the case of Fig. R18). If we tilt the zone axis of CoO nanorod to the direction shown in Fig. R19b to see the Ru/CoOx interface, the projected nanoparticles look like a shell.

Original comment 4-15:
Considering the fact that interfacing of RuO2 with CoOx leads to Ru atoms with a higher electron density (cf fig 1d and 2e), this seems to be good for improving the stability but how is it better for improving the oxidation of water of hydroxide ions? I might miss something but Co charge does not change and the mechanism is said to occur at the interface (Ru/Co dual-atom site) between the two materials.

Response:
Thanks for your comment. In RuO2/CoOx, the electron interaction among the face-to-face Ru-O-Co interfacial atoms enhance the stability, while the Ru/Co dual-atom site exposed around the interface is responsible for the improved activity. Our DFT calculation reveals that the oxygen 31 intermediates are co-adsorbed at the Ru/Co dual-atom site (Fig. R20a). As discussed in the main text, the co-adsorption configuration facilitates the stabilization of key intermediate *OOH at the Ru/Co dual-atom site around the interface and transforms the OER mechanism with a significantly decreased energy barrier compared with RuO2 (Fig. 20b). This is verified by our electrochemical tests and KIE results, as discussed in the main text. Moreover, to experimentally verify interfacial Ru/Co dual-atom sites as the most active OER sites, we have performed additional experiments. First, we deposited RuO2 nanoparticles on carbon black with the same particle size (~2 nm) as that on RuO2/CoOx ( Fig. R21a and R21b). The higher current density of RuO2/CoOx compared with that of RuO2 with the same RuO2-mass (Fig. R21c) indicates the crucial role of the interface in enhancing OER performance. Moreover, we prepared RuO2/CoOx with varied RuO2 particle size or loading masses (Fig. R22a). Assuming either surface Ru site or interfacial Ru-Co dual-atom site as the most active sites, the corresponding site number was calculated and correlated with the OER current density at 1.50 VRHE ( Fig. R22b and R22c). An adequate linear relationship was observed between the number of interfacial Ru-Co dual-atom site and the OER current density (Fig. R22c). These collective results reveals that the interfacial Ru/Co dual-atoms are the most active OER sites.

Response:
Thanks for your comment. We constructed the interface using the lattice parameters of RuO2 and CoO from the Crystallographic database (Fig. R23). As shown, there is good lattice match between RuO2 and CoO. During the synthetic process, the interfacial strain was fully relaxed by treating the catalyst in N2 flow at 400 ℃ for 0.5 h. We observe no evident strain and defects at the interface.
33 Figure R23. Interface model of RuO2/CoOx. Fig. 1a and 1b are a bit deceiving in this sense because one goes to pH = 0 and the second only to 7: the calculations are not valid when pH<7?) Is this strategy not applicable at low pH? The system seems anyway to be a bit complex to be included in a real electrolyzing cell and dissolution of the CoOx in low pH for instance may degrade the membrane.

Response:
Thanks for your valuable comment. We agree with the Reviewer that CoOx can be dissolved in low pH and current RuO2/CoOx system cannot be directly applied in low pH environment. However, the strategy proposed in this work is applicable. Actually, we are exploring oxide support material (such as WOx), which can be preferentially oxidized instead of RuO2 in acidic solution. We hope to share our results in future work. Besides, we note that OER is also a very important half reaction for carbon dioxide reduction, which is preferable for operating in neutral environment (Joule 2021, 5, 737). However, the progress of neutral OER was far behind compared to the well-developed acidic and alkaline OER. We hope that our current developed RuO2/CoOx catalysts, which shows an outstanding activity and stability under neutral conditions can be applied in these occasions.
In response, we have added some outlook in lines 18-22, page 9 in the revised conclusion. Response: Thanks for your valuable comment. Accordingly, we have listed the positions of *OOH bands in the literature in (Table R3). The difference between the wavelength of the *OOH bands in our samples and the reported values is due to the pH effect. We measured the in situ surface-enhanced IR spectra of CoOx in both 1.0 M PBS and 1.0 M KOH (Fig. R24). As seen, the position of *OOH bands in 1.0 M KOH (1050 cm -1 ) is very close to the reported value in CoOx (Table R3). Moreover, the *OOH bands of CoOx shift toward lower wavelength direction in alkaline solution compared with that in neutral solution (the more evident *OOH bands in alkaline solution is due to the higher OER activity, yielding more *OOH). We note that the location of *OOH on our samples in neutral solution is between the reported values of *OOH in alkaline and acidic solutions (   Yes, the more pronounced *OOH indicates the easier formation of *OOH on RuO2/CoOx compared with that on pristine RuO2. This is supported by our calculation result that the ratedetermining step on RuO2/CoOx shifts to the subsequent process (O2 desorption).
In response, we have added Fig. R25 as Supplementary Fig. 29 in the revised supporting information. Moreover, we have added

Response:
We would like to thank the Reviewer for his/her positive recommendation and valuable comments to help us to improve the quality of this manuscript.

Original comment 1-2:
It is hoped that the authors are planning further studies that investigate issues like stability under dynamic current/voltage loading as well as with higher current density than 150 mA/cm² in neutral pH as well as in acidic pH.

Response:
Thanks for your kind suggestion. Accordingly, we have tested the stability of RuO2/CoOx with varied current densities from 10 to 100 mA cm −2 in 1.0 M PBS (Fig. R1). As shown, RuO2/CoOx shows excellent dynamic stability. Moreover, following the suggestion of the Reviewer, we have increase the loading mass of RuO2/CoOx to achieve higher current density. Please see our response to your comment 4-2. Figure R1. Dynamic stability test of RuO2/CoOx with current densities from 10 to 100 mA cm −2 in 1.0 M PBS.
In response, we have added Fig. R1 as Supplementary Fig. 19 in the revised supporting information. Moreover, the above related discussion on the dynamic stability of RuO2/CoOx has been added in lines 22-24, page 6 in the revised manuscript.
"Significantly, the RuO2/CoOx catalyst works stably at a constant current density of 10 mA cm -2 for more than 200 h (Fig. 3f), and affords an excellent dynamic stability with varied current density from 10 to 100 mA cm -2 ( Supplementary Fig. 19)."

Original comment 2:
Abstract: The concept is only vaguely described as ""artificially constructed a RuO2/CoOx interfaceit is not clear what this means. Also the pH range which in the stabilisation and enhanced activity were verified is not specified. Moreover, no quantitative data was provided for these verifications.

Response:
Thanks for your valuable comment. Accordingly, Abstract in the revised manuscript has been rewritten in lines 20-22, page 1, "Here, we report breaking the stability and activity limits of RuO2 in neutral (pH=7)

Response:
Following the suggestion of the Reviewer, one new sentence has been added in lines 2-3, page 3 in the revised Introduction to discuss reducing the use of Ru for the wide application of electrolysis in the future.
"Moreover, the interface construction may use some cost-effective materials to reduce the use of precious metal Ru and achieve sustainable water electrolysis." Original comment 4-1: Methods: The article cited by the authors as a reference for the procedure for production of the CoOx samples, itself also cites another paper (https://doi.org/10.1038/ncomms12876). Please update appropriately.

Response:
Thanks for your suggestion. The reference the Reviewer mentioned has been added as Reference [38] in the Methods and in the Reference list in the revised manuscript.

Original comment 4-2:
Why didn't the authors go higher than 150 mA/cm² in neutral solution?

Response:
Thanks for your valuable suggestion. We have used nickel foam to load RuO2/CoOx with a higher mass of 1.5 mg cm -2 . As shown in Fig. R2, the RuO2/CoOx can achieve a current density up to 400 4 mA cm -2 at 1.92 VRHE in neutral solution. Figure R2. OER polarization curve of RuO2/CoOx with a catalyst loading of 1.5 mg cm -2 in neutral electrolyte (1.0 M PBS).
In response, we have added Fig. R2 as Supplementary Fig. 29 in the revised supporting information. Moreover, the above related discussion has been added in lines 10-11, page 7 in the revised manuscript.

Original comment 4-3:
What was the Ru loading of the commercial catalyst used for comparison in Supp. Fig 26? Response: Thanks for your kind noticing. The loading mass of RuO2 for comparison in Supplementary Fig.   27 (original Supplementary Fig. 26) is 0.255 mg cm -2 . In response, we have specified the loading mass of RuO2 in the caption of Supplementary Fig. 27.
"It shows that the catalyst with RuO2 loading of 84 µg cm -2 exhibits the optimal OER activity, which is better than the commercial RuO2 catalyst (0.255 mg cm -2 )." Original comment 5-1:  Fig 34, is rather simplistic in reality, one should also consider the total "active or catalytically relevant" element loading since Co seems to also play a role in the catalytic activity.

Response:
Thanks for your kind suggestion. Accordingly, we have compared the current densities of CoOx, and RuO2/CoOx, RuO2 with varied RuO2-masses at 1.65 VRHE (Fig. R3a). Moreover, we have added the polarization curve of the CoOx in Fig. R3b. These results show more clearly the role of pristine CoOx, RuO2 and RuO2/CoOx interface in catalysing OER, as suggested by the Reviewer. curves of the RuO2/CoOx with varied RuO2 particle size or loading masses and CoOx in neutral electrolyte. We note that the orange, blue, and dark brown curves represent RuO2/CoOx (10 µgRuO2) with RuO2 particle size of 2, 3 and 4 nm, respectively.
In response, Fig. R3a and Fig. R3b was added as Supplementary Fig. 37b and Supplementary   Fig. 39a in the revised supporting information. Moreover, the related discussion have been added in the caption of Supplementary Fig. 37.
"Note that the current densities of RuO2/CoOx are significantly higher than those of RuO2 and CoOx, indicating that the RuO2/CoOx interface is the key to improve the OER performance of RuO2/CoOx."

Original comment 5-2:
Extend the caption of Supp Fig 36(a) to help the reader distinguish which particle size each of the various curves with a Ru loading of 10µg relates to.

Response:
Thanks for your kind suggestion. In response, we have added the relative description suggested by the Reviewer in the caption of Supplementary Fig. 39a (original Supplementary Fig. 36a) in the revised supporting information.
"We note that the orange, blue, and dark brown curves represent RuO2/CoOx (10 µgRuO2) with RuO2 particle size of 2, 3 and 4 nm, respectively." Original comment 5-3: In Supp Tables 5 & 7, it would also be useful to present stability data if possible because that is the main theme of this manuscript.

Response:
As suggested, stability data have been added in Tables R1 and R2. In response, Supplementary Tables 5 and 7 have been updated with Tables R1 and R2 in the revised supporting information.